1. Field of the Invention
This invention relates to networking and, more particularly, to link aggregation within a network.
2. Description of the Related Art
Link aggregation is used to logically combine two or more individual links into a single aggregated link. Link aggregation can provide improved performance and increased fault tolerance. Improved performance arises because the aggregated link appears to have a bandwidth equal to the combined bandwidth of the individual links. Traffic can be load-balanced among the individual links. Increased fault tolerance is provided since one or more individual links within an aggregated link can fail without disrupting communication between the devices coupled by the aggregated link. Link aggregation techniques include Link Aggregation Control Protocol (LACP), which is defined in IEEE 803.2ad, and Port Aggregation Protocol (PAgP), which is a standard promulgated by CISCO SYSTEMS, INC.
Typically, aggregated links are established between two devices. These devices communicate with each other according to a link aggregation protocol in order to determine whether any of the links between the two devices can be operated as an aggregated link. Typically, communication according to the link aggregation protocol takes place using Protocol Data Units (PDUs). Each device includes an identifier, which uniquely identifies that device for purposes of the link aggregation protocol, in PDUs sent by that device. If a device receives PDUs on two different links, and each of the received PDUs includes the same identifier, the device determines that both links are connected to the same partner device. Accordingly, the device can operate those two links as an aggregated link with the partner device.
In order to provide improved network fault-tolerance, field-replaceable components (i.e., components that can be replaced while the equipment is in the field) are often used within networks. For example, network devices can be implemented with multiple field-replaceable line cards. If one field-replaceable line card fails, that line card can be replaced without having to replace the entire network device. Similarly, in a stackable switch, several switches can be interconnected such that the switches act as a single device. If one switch fails, that switch can be replaced without having to replace the other switches within the stackable switch.
When link aggregation is used with devices that include multiple field-replaceable components and it is desired that links going to different field-replaceable components be able to form aggregated links with each other, all of the field-replaceable components that make up the same device must use the same identifier in aggregation protocol PDUs. Otherwise, a partner device would think that each field-replaceable component was a separate network device, and aggregated links would not be formed for links coupled to different field-replaceable components. Typically, one of the field-replaceable components supplies the identifier to all of the other field-replaceable components.
This above arrangement works well, unless the field-replaceable component supplying the identifier is replaced (e.g., due to a failure within that component). At that point, the remaining field-replaceable components within the device continue to use the identifier supplied by the failed field-replaceable component. If the failed field-replaceable component is repaired and replaced in a different part of the network (and thus is no longer part of the device using the identifier), two different devices, the device that used to include the field-replaceable component, and the field-replaceable component, may inadvertently include the same identifier in link aggregation PDUs. If both devices are coupled to the same partner device, an aggregated link can be formed on links coupled to both devices. Since this aggregated link includes links that terminate on two different devices that are operating independently of each other, improper operation may result. Accordingly, it is desirable to be able to be able to handle situations in which the field-replaceable component supplying the identifier is removed or relocated within the network.